The present invention pertains to a process for forming optoelectronic semiconductor devices, particularly, to a process for forming an improved semiconductor laser and to inventive semiconductor lasers formed using such process.
A semiconductor laser produces laser light when an appropriate voltage is applied. Semiconductor lasers are currently used in a wide variety of applications depending upon their output power and wavelength. A typical application for semiconductor lasers is in the field of telecommunications.
When used in telecommunications, lasers, combined with other semiconductor devices, transmit data, video, and audio information from one point to another. In designing such a transmission system it is desirable to maximize the distance between the point of transmission and point of reception. In general, the greater the distance that can be achieved between a transmitter and a receiver the less expensive is the system.
For instance, video information can be transmitted a distance of 3,000 miles in a number of ways. The preferred way would be to use a single transmitter utilizing a single laser at one end and a single receiver at the other. Lasers which are currently used in transmitters, however, can only send information over relatively short distances. As a result, electrical/electronic relays or "repeaters" are needed every 30 or 50 miles to regenerate the original signal in order to avoid losing any of the information that was originally transmitted. Thus, to transmit over a distance of 3,000 miles the installation of many repeaters is required. The installation of these repeaters is expensive and also introduces undesired complexities which reduces the overall reliability of the system.
It can thus be understood that the development of a laser based transmitter which can operate over long distances would result in the elimination of many repeaters, the net result being a cost savings to the system.
The distance over which information can be transmitted using lasers is determined by the physics of the laser itself and the optical fiber through which the laser light travels. Generally, it has been found that certain types of lasers, namely single mode lasers, are best suited for the transmission of information over great distances. Examples of such single mode lasers are distributed feedback (dfb) and distributed Bragg reflective (dBr) lasers.
In addition to a laser, a semiconductor-based transmitter must include a means of modulation along with an appropriate grating, e.g. dfb or dbr type gratings. The grating determines the output wavelength of any laser light emitted from the transmitter. Once a particular wavelength of light is selected, e.g. 1.554 microns, the information sought to be transmitted must be placed, impressed or modulated onto this lightwave "carrier", i.e. a lightwave with no information on it. Modulators are used to place an information pattern on the lightwave carrier. This information pattern directly corresponds to the information that is to be transmitted.
There are three common ways to modulate a lightwave carrier. First, the modulation may be direct, by modulating the laser driving current. Second, it may be "externally-discrete" in which the light from the laser is passed through a physically separated modulator. Finally, modulation may be "externally-integrated"; in which the modulator, while separate from the laser functionally is physically fabricated on the same semiconductor substrate. The latter two modulation schemes are usually referred to as external modulation.
Direct modulation results in wavelength as well as amplitude modulation. External modulation offers the possibility of much reduced or zero wavelength modulation. Of the two methods of external modulation, externally-integrated modulation is preferred over externally-discrete modulation for several reasons. The tradeoff, however, is that the placement of a modulator on the same semiconductor substrate as a laser, to form a so-called integrated laser/modulator ("ILM"), creates additional technical hurdles that must be overcome if the information pattern is to be correctly transmitted over a long distance.
It is imperative that phenomena called "chirping" and any related "dispersion" be limited or reduced to allow for proper transmission and reception and this in turn requires a high degree of electrical and optical isolation between the laser and modulator.
As will be recognized by those skilled in the art a certain amount of laser light of a given wavelength will not be output from the ILM. Rather, some portion of this light will be reflected; won't escape the output facet of the ILM due to "deviations" in the anti-reflective coating ("AR") placed thereon. It will be understood by those skilled in the art that a perfect AR coating, while impractical in mass-production applications, would result in zero reflections. Any "deviation" in this coating from that of a perfect AR coating causes reflections to occur. Such reflected light can cause adverse effects on the operation of the ILM. One such effect is "chirping".
Chirping occurs when this reflected light acts to change the resonant operating frequency of the laser which in turn directly acts to change the output wavelength of the ILM. Instead of outputting a continuous, narrow wavelength of light at 1.554 microns, chirping may result in the output of differing wavelengths of light, e.g. 1.553 microns, 1.555 microns, or the broadening of the desired narrow wavelength, e.g. light output encompasses both 1.554 and 1.555 microns.
The need for maintaining a certain wavelength of light, e.g. 1.555 microns, is due to the fact that the optical fiber through which such light will travel is designed to transmit light at a certain wavelength. For example, if other wavelengths of light are transmitted through an optical fiber designed to transmit light at a wavelength of 1.555 microns, such other wavelengths of light will not travel as fast through such fiber as a lightwave having a wavelength of 1.555 microns. As a result each wavelength of light which is transmitted at the same time may be received at different times instead of all at once. The delay between the arrival of differing wavelengths of light, each containing a piece of the information pattern, may result in the information pattern being unrecognizable.
This physical property of an optical fiber which results in its passing certain wavelengths of light faster than others is referred to as dispersion. It will be recognized by those skilled in the art that a reduction in chirping will result in a decrease in a given systems' "dispersion penalty" due to pulse broadening from chirping of an ILM, which in turn reduces signal deterioration.
Accordingly, it is the object of the present invention to produce an integrated laser/modulator with reduced chirping characteristics.
Another object of the present invention is to provide a repeatable and cost-effective process for mass producing integrated laser/modulators with reduced chirping characteristics.
Still another object of the invention is to provide a process for controlling the physical dimensions of an integrated semiconductor laser/modulator in order to reduce the effects of any unwanted chirping.